The science instruments are the heart of the James Webb Space Telescope. The four science instruments are contained within the Integrated Science Instrument Module or ISIM. JWST's instruments will detect light from distant stars and galaxies, and planets orbiting other stars.
The ISIM structure (pictured below) is like a chassis in a car providing support for the engine and other components.
JWST Team Photo with completed Flight Instrument module:
Many different smaller parts go into the whole ISIM structure. These include the ISIM Structure Subsystem; ISIM Thermal Control Subsystem; ISIM Control and Data Handling Subsystem; ISIM Flight Software; and ISIM Harness Assemblies.